Vehicle headlamps commonly include a lamp capsule mounted in a reflector so that the light source is located at or near the focal point of the reflector. Light emitted by the lamp capsule is directed in a forward direction by the reflector. The lamp capsule typically includes a high beam filament from which light is directed horizontally in a high beam pattern and a low beam filament from which light is directed below horizontal in a low beam pattern. One of the problems involved in the design and construction of vehicle headlamps is to minimize uncontrolled light emission outside the beam patterns, particularly the low beam pattern, that may impair the ability of oncoming drivers to see the road and other vehicles. This uncontrolled light is known as glare.
The lamp envelopes of lamp capsules used in vehicle headlamps are commonly formed from tubes that are domed, or tipped off, at the forward end and are press sealed at the rear end. The lamp capsule is aligned in a headlamp reflector so that the domed portion faces forward and the axis of the lamp envelope is colinear with the optical axis of the reflector. Light emitted from the light source within the lamp envelope projecting directly forward encounters the domed portion. Due to the irregular shaping of the dome, this light is refracted at odd angles and becomes a hot spot or glare source. To control the irregular light emission, the dome of the lamp envelope is commonly dipped in a black paint to block light from exiting the lamp in the domed portion of the lamp envelope. An example of a prior art vehicle lamp capsule having a press seal and a dome with a radiation-absorbing coating is disclosed in U.S. Pat. No. 4,794,297, issued Dec. 27, 1988 to Gaugel et al.
It had been believed by workers in the field that the black paint absorbed most of the light, thereby solving the glare problem. However, it has been found that a noticeable portion of the light expected to be absorbed in the dome may be reflected back into the lamp, striking the light source, the support structures and the press seal. This reflected light is then reflected out of the lamp, where it remains uncontrolled and produces glare. Accordingly, there is a need to control the light projected into the dome of a vehicle lamp capsule.